1. Field of the Invention
The present invention relates to an ignition coil for installation inside an engine plug-hole.
2. Related Art
A conventional stick-type ignition coil includes primary and secondary coils rolled around two spools having respective different diameters and a bar-like center core. The primary and secondary coils and the center core are concentrically installed inside a cylindrical coil casing. A high-voltage terminal connectable to an ignition plug is attached to the lower end of the coil casing by adhesive. Filler such as epoxy-based thermosetting resin fills the upper otherwise open end of the coil casing. In this conventional ignition coil, as shown in FIGS. 9A and 9B, a high-voltage terminal 1 is formed at one coil and by bending copper wire into a U-shape, for reducing manufacturing cost. This high voltage terminal 1 is vertically press-inserted into a terminal support 2 integrally formed at the lower end of the secondary spool 5. Further, a pin-shaped central high-voltage terminal 3 protruding upwardly from a high-voltage tower portion (not illustrated) is upwardly inserted into and connected to the U-shaped high-voltage terminal 1. Here, the terminal support 2 has a circularly-shaped terminal insertion hole 4 into which the high-voltage terminal 3 is upwardly inserted.
However, as shown in FIG. 9B, when the high-voltage terminal 3 is pinched by the U-shaped high-voltage terminal 1, both sides of the high-voltage terminal 3 are expanded to a certain degree to attain sufficient contacting pressure therebetween. In this condition, the resilient forces of both sides of the high-voltage terminal 1 push the high-voltage terminal 3 toward the opening side of the U-shaped high-voltage terminal 1 as denoted by arrow A. When the high-voltage terminal 3 is inserted into the high-voltage terminal 1, the coil casing is not yet filled and the lower end of the secondary spool 5 (terminal support 2) is not fixed. Thus, the lower end of the secondary spool 5 slides toward the opposite side of arrow A to an eccentric position due to the reaction force acting on high-voltage terminal 3 opposite arrow A. Therefore, the desired concentricity between each component inside the coil casing is reduced and the electrical insulating distance therebetween varies, thereby reducing the degree of insulation between components.
In this case, as the maximum offset of the secondary coil 5 is defined by clearance B between the terminal insertion hole 4 and the high-voltage terminal 3, the offset of spool 5 can be reduced by making clearance B small. However, when clearance B is made small, it becomes more difficult to insert high-voltage terminal 3 into hole 4, and the assembling process becomes less desirable. That is, in the above-described conventional high-voltage terminal connection structure, it is difficult to simultaneously attain both high accuracy distances between assembled parts (insulating performance) and efficient, relatively easy assembly processes.
JP-A-8-213259 discloses another conventional stick type ignition coil. This ignition coil includes, as shown in FIG. 10, a bar-like center core 102, a secondary coil 104 rolled around a secondary spool 103 disposed at the outer side of center core 102, a primary coil 106 rolled around a primary spool 105 disposed at the outer side of secondary coil 104, and an outer core 107 disposed at the outer side of primary coil 106. A thermosetting resin fills the gaps between these components to attain electrical insulation and mechanical strength inside housing 101.
In general, an ignition coil needs to be installed in a restricted space like an engine plug-hole in which the coil portion outer diameter is less than 24 mm. Thus, permanent magnets 109, 110 need to be disposed at both ends of center core 102 for generating required ignition coil voltage. Here, the excitation poles of permanent magnets 109, 110 are opposite to the polarity of center core 102.
A rare-earth magnet such as neodymium is used for permanent magnets 109, 110, so as to generate sufficiently high magnetic force in the restricted small space. The need for permanent magnets 109, 110 increases manufacturing cost for the ignition coil.